1. Field of the Invention
The present invention relates to coating of substrate webs with polyethylene.
2. Description of the Prior Art
Processes for extrusion coating of substrate webs with polyethylenes are known. One problem, associated with the coating material, which is well recognized in the art, is a phenomenon known as necking-in. Necking-in occurs after the molten polyethylene is extruded through a slotted die. The edges of the molten sheet of polymer have a tendency to move inwardly towards the center of the sheet, thus making the width of the sheet narrower than when it was first extruded from the die slot. As a result of this necking-in, the thickness of the edges of the polymer sheet, when solidified, tends to be greater than that at the center of the sheet. The thickness at the edge may be ten- or twenty-fold that at the center. An objectionable effect of this edge thickening of the coated web is to increase the diameter, at the edges, of a beam wound with the web, which leads to tensioning and wrinkling difficulties.
Several techniques have been used to overcome this objectional phenomenon.
A common method involves trimming the thickened edges from the coated web. This method is wasteful, as in many instances the coating material and the substrate to which it is attached are dissimilar, and recovery of the trimmed coating material and substrate is difficult.
Another method is disclosed in U.S. Pat. No. 2,894,853, issued July 14, 1959 to N. Andersson in which the edge of the substrate web, e.g., paper is thinned by grinding prior to coating the web with the coating material.
Yet another method involves blending polyethylene resins, each having certain desirable properties. Canadian Patent No. 653,596, issued Dec. 4, 1962 to R. T. Buckso, describes such a blend, which comprises 30-70%, by weight, of a polyethylene resin having a density of from 0.92 to 0.945 g/cm.sup.3, and a stiffness modulus of about 30,000 to 55,000 psi and a melt index of about 2 to 8 dg/min, and 70-30%, by weight, of a polyethylene resin having density of from 0.95 to 0.97 g/cm.sup.3, a stiffness modulus of from about 85,000 to 200,000 psi, an elongation modulus of at least 1,000% at break and a melt index of 1 to about 6 dg/min.
Heretofore, the requirements of a polyethylene coating resin have included the ability to be extruded and drawn down to the substrate web without tearing or substantial necking-in of the molten sheet extrudate. Such requirements are similar to those required for polyethylene resins intended for casting films. Melt flow properties of such polyethylene resins as indicated by melt index, tend to be in the range of 3.5 to 12 dg/min. Polyethylene resins having melt indices of 30 dg/min and above have, heretofore, been believed to be unsuitable for coating processes primarily because when extruded into sheet form such resins neck-in to a much greater degree than do polyethylenes having substantially lower melt indices. As used herein, melt index refers to measurements made according to the procedure of ASTM D-1238 (Condition E).
It is occasionally desirable to impart stiffness to webs. One method for stiffening webs that has been suggested is to coat the web with a high density polyethylene. An added advantage of using a high density polyethylene coating is that high density polyethylenes draw down to a greater extent than do low density polyethylenes. Such greater draw down lends itself to the provision of thinner coatings. In spite of the benefits of greater draw down and a stiffer coated web, high density polyethylenes have not been used on a large commercial scale in extrusion coating processes because of the increased tendency of such extruded resins to neck-in.
The term "draw down" as used herein refers to the ratio of the thickness of the coating material immediately after extrusion from the die slot to that after being laid down on the substrate.
Yet another problem is that in conventional extrusion coating processes where the draw down of the extruded coating material may be as high as 20:1 or higher, the phenomenon of necking-in tends to be more pronounced than in slower speed processes. Coating processes may operate at substrate web speeds of as low as about 50 m/min, higher speed processes are desirable for reasons of productivity, and speeds of 200-400 m/min or higher are not uncommon. Conventional processes tend to be operated at these higher speeds.
It appears that prior processes have not been entirely successful in overcoming the problems associated with the necking-in phenomenon in extrusion coating processes.
It is therefore an object of the present invention to provide an extrusion coating process in which a substrate web may be coated with a polyethylene, and in which the edge thickening is minimized; and in which other processing difficulties caused by the necking-in phenomenon are minimized.